The potential oxidation characteristics of CaCr₂O₄ during coal combustion with solid waste in a fluidized bed boiler:...

CaCr₂O₄ (Cr (III)), mainly generated through the decomposition of CaCrO₄ (Cr (VI)), is a significant intermediate for toxic Cr (VI) formation during solid fuel combustion. In this study, the formation, oxidation and sulfation kinetics of CaCr₂O₄ were analyzed to forecast the potential of CaCr₂O₄ oxidation during co-firing of coal and solid waste in a circulating fluidized bed boiler. The results indicated that the formation and oxidation of CaCr₂O₄ were fitted to a single step nucleation and growth model while CaCr₂O₄ sulfation was fitted to a shrinking core model. CaCr₂O₄ formation through CaCrO₄ decomposition was favored in oxygen-lean atmosphere and considerably suppressed in the presence of oxygen. In contrast, CaCr₂O₄ oxidation was mainly determined by the contacts between CaCr₂O₄ and CaSO₄/CaO, which influenced not only oxidation rates but also the product species. Moreover, the oxidation reactivity of CaCr₂O₄ was higher in the presence of CaO than that of CaSO₄. On the other hand, CaCr₂O₄ sulfation can occur more easily than CaCr₂O₄ oxidation, the reaction rate of which was deeply affected by sulfate product layer. Findings in this study suggested that spraying calcium in furnace for desulphurization may increase the risk of CaCr₂O₄ oxidation. Measures including the adjustment of Ca/S ratio in blended fuel (with added limestone) and operating conditions (such as temperature and local atmosphere) in co-firing system could be taken to control CaCr₂O₄ formation and oxidation.